The long-term objective of the proposed studies is an enhanced understanding of the cellular mechanisms of nociception and analgesia. The transmission of noxious stimuli from peripheral nociceptors to the spinal cord is dependent in large part on the interaction of substance P (SP), a peptide released from sensory nerve endings, and substance P receptors (SPR) which are expressed by neurons in the spinal cord and dorsal column nuclei (DCN). Recently, we have shown that SP release from primary afferents drives SPR internalization in spinal cord neurons and provides a specific image of the cells activated by SP in the spinal cord in vivo. Building on this and related observations we will: (1) Determine the somatosensory stimuli that induce SP release in the spinal cord and DCN in the normal rat, determine how different anesthetic agents influence the release of SP and the SP-induced SPR internalization, and determine whether morphologically distinct classes of SPR- immunoreactive neurons and glia in the spinal cord and DCN are activated by different modalities of somatosensory stimulation. (2) Determine the neuronal and glial cell types that show an up- regulation of the SPR after nerve injury or in an inflammatory pain state. (3) Determine whether, after nerve injury or in inflammatory pain, there is an increased release or diffusion of SP in the spinal cord or DCN in response to noxious and non-noxious stimuli or nerve stimulation and whether an additional subset of neurons and glia is now activated by SP released from primary afferents. (4) Determine whether opiates, prostaglandins and other ligands that target receptors expressed on the presynaptic terminals of primary afferent neurons differentially modulate the release of SP from sensory neurons in the normal animals vs. animals with nerve injury or persistent inflammatory pain. (5) Using similar techniques that we have successfully employed to raise antibodies to the SPR, raise antibodies that recognize; a subtype of the SPR that has a truncated C-terminus, the neurokinin-2 and neurokinin-3 receptors and determine what role these tachykinin receptors play in nociception and analgesia. The goal of the proposal is to explore the cellular basis of pain perception and discover targets in the signal transduction pathway that may be the most effective points for interventions in the control of pain.